There are two ways Google Smart Lock works - Smart Lock for Android and Smart Lock for Passwords.

Smart Lock for Android is a setting on your Android phone that allows you to setup safe Places, Devices, Faces and even voices to make it faster to unlock your Android phone where you feel you need less security. You need at least Android 5.0 (Lollipop) to access these features

Why is this feature cool? It can make accessing your phone much quicker to access, whilst still retaining a good level of security on your phone if you are out and about

Smart Lock for Passwords helps you securely store passwords to some of your favourite websites and compatable apps. An example app that can use this feature is Netflix

Why is this feature cool? By turning this feature on in your Chrome Browser on desktop and Android phone, Smart Lock auto remembers passwords so you don't have to remember them. It also sync's across all of your Android devices. Smart Lock for passwords allows you to have different passwords for all sites or apps, which should increase your overall level of online security. As opposed to using the same password for all sites or apps. Very risky